Comparison of Factor Correction Techniques for Generator-Sets for SHEVs Ahmed Al-Busaidi, Dimitrios Kalpaktsoglou, Volker Pickert Newcastle University, School of Electrical, Electronic and Computer Engineering, Newcastle upon Tyne, NE17RU, UK E-mail: volker.pickert@ncl.ac.uk Copyright 2009 MC2D & MITI Abstract: The paper presents a comparison of conventional and unconventional power (PF) correction techniques applied to generator-sets operating in series hybrid electric vehicles (SHEV). Conventional PF correction techniques such as PWM operated rectifiers run at high switching frequencies resulting in high switching losses. Controlled series compensator (CSC) rectifiers, which are less known in automotive and therefore here regarded as unconventional, promise reductions in switching losses without power degradation. The operation and performance of various topologies for both conventional and unconventional PF correction techniques is discussed and simulated using Matlab/Simulink. Simulation results in terms of the first harmonic of the phase current, total harmonic current distortion (THD), PF, and output is provided. Keywords: correction techniques, CSC, SHEV. 1. Introduction SHEVs have become very popular as they promise fuel benefits as well as reduction in emissions [1, 2]. The power drive train of a SHEV is simple. The wheels are driven by an electric motor, powered by a battery with an engine plant that cranks a generator unit to provide electric power to the battery and if necessary to the electric motor. One of the major components of a SHEV is the generator unit. Generally, the generator unit consist of a 3-phase machine and a 3-phase rectifier circuit. Rectifier circuits convert the alternating machine output to dc to store the energy to the battery. This paper evaluates four conventional rectifier topologies suitable for SHEVs: uncontrolled full-bridge diode rectifier, controlled full-bridge Thyristor rectifier, uncontrolled full-bridge diode rectifier with dc/dc boost converter and PWM -source current controlled rectifier. Previous studies [3] have shown that none of the conventional circuits provide high PF and high output in order to increase the overall efficiency of the power drive train. There are two reasons for that: 1) there is always a drop across the machine inductance and 2) the series connection of the machine inductance with the rectifier topology leads to a commutation effect within the rectifier which distorts the phase s and the phase currents. The result of this distortion is a reduction in the first harmonic of the machine and machine current which consequently leads to a reduction in the PF. One way to improve the PF is to compensate the impact of the machine inductance by using CSC converters. CSCs are capable to increase the PF and provide high output, maximising the efficiency and the performance of hybrid electric
vehicles. This paper compares PF correction techniques used in conventional rectifier circuits and CSCs. A brief definition of CSC converters is given in the next section. 2. Controlled Series Capacitor (CSC) Converters CSCs are not new topologies and they have been used in power transmission lines to control the reactive power. CSC s circuits have been simulated also for Wave Energy Conversion (WEC) buoys [4], giving excellent results. On these systems the PF greatly improved, maximising their overall efficiency. In WEC systems the output varies in amplitude and frequency which is a similar incident to the output signal produced by the generator unit in a SHEV. CSCs operate at much lower switching frequencies promising a reduction in switching losses, without PF degradation. CSCs principally consist of a variable capacitor placed between the generator and an uncontrolled full-bridge diode rectifier. The capacitance of the system is then controlled with respect to the electric frequency, in order to match the inductance of the generator. Therefore, any inductive reactance can be compensated as long as the inductive reactance X L is equal to the capacitive reactance Xc. At the resonant point the effect of the machine inductance is eliminated and the system s PF become unity. The three investigated CSC converter circuits are called: thyristor-switched series capacitor (), switched variable capacitor (SVC), and forced commutation controlled series capacitor (FCSC). The performance of these three converters and the four rectifiers, given in the introduction, have been evaluated, using MATLAB/SIMULINK environment, in terms of PF, output, (THD), and power losses. A 50kW three-phase permanent magnet synchronous generator (back EMF 325V, Ls=1.407mH, Rs=0.06Ω) was simulated to feed the rectifier circuit. 3. Thyristor Switched Series Capacitor () The circuit, shown in Fig.1, consists of a number of capacitors in series, each shunted by a switch, composed of two anti-parallel thyristors [5]. All capacitors have the same value C. The battery and the electric drive of the SHEV power drive train is shown as a capacitor and a resistive load for simplicity. The machine is represented by the back EMF V, inductance L and resistance R. Figure 1: Thyristor-Switched Series Capacitor () The overall capacitance is controlled by conducting or blocking each of the thyristor pairs. If a thyristor pair conducts, the capacitor C is short circuited. If a thyristor pair is open, the value C is added to the total capacitance C T. The total capacitance of the circuit is given by: C T = C / m (1) where m is the number of active capacitors. If all capacitors are bypassed the equivalent capacitance becomes C T = 0 F. In order to correct the PF, X C should be equal to the reactance X L : 2 X C = X L 1 ω CT = ωl CT = 1 ω L (2) The desired number of active capacitors for unity PF for every frequency can be calculated using equations (1 ) and (2) 2 2 C m = 1 ω L m = C ω L (3) Since C and L are fixed, m is directly related to the electric frequency f. All the thyristors are commutated naturally and they turn off when the current crosses zero. At this time a capacitor can be inserted into the line as shown in Figure 2. Figure 2: Capacitor insertion at zero current Once the capacitor is in line, it will be charged to its maximum value, during the full halfcycle of the line current and discharged from its maximum to zero during the negative line current cycle [5,6].
4. Forced-Commutation Controlled Series Capacitor (FCSC) The FCSC is shown in Fig. 3 consists of a capacitor and a pair of switches, such as IGBTs for example, each connected anti-parallel [5]. By choosing the appropriate delay angle, the system operates at its resonant point and the PF become unity. 5. Switched Variable Capacitor (SVC) The SVC rectifier circuit consists of two parallel capacitors, C 1 and C 2, which are connected with two switches, S 1 and S 2, as shown in Fig. 4. Figure 3: Forced Commutation Controlled Series Capacitor (FCSC) The principal target in this case is the same as previously therefore the effects of inductive reactance should be eliminated. The capacitive reactance X C can vary from X = 0 to X C = 1 ωc in order to match any inductive reactance. When the IGBT switches S 1 and S 2 are closed, the capacitor bank is short circuited. When the switches are open the current flows through the capacitor C FCSC. Therefore, the capacitor can be controlled by closing and opening the switches each half-cycle. The series reactance can be expressed as: VC 1 X C = = ( 1 ( 2 π ) γ ( 1 π ) sin γ )(4) ( γ ) 2 I ωc where is the delay angle [5]. The graph below shows with the red line the inductive reactance for frequencies up to 350 Hz. The blue lines represent the capacitive reactance for various delay angles over frequencies up to 350 Hz. Capacitive and Inductive Reactance (Ohm) 6000 5000 4000 3000 2000 1000 Capacitive & Inductive Reactance against Frequency C Figure 4: Switched Variable Capacitor (SVC) The values of capacitors C 1 and C 2 have been chosen carefully to match the inductive reactance at low, and high frequencies. Therefore, capacitor C 1 will attempt to hit the resonant point at high frequencies, while capacitor C 2 was chosen for low frequencies. The two switches alternate in their switching status and the average capacitance C av becomes a function of the duty [7]. In order to alternate the switching status PWM technique is used. This allows the change of the average capacitance. When the frequency is low, the inductive reactance becomes maximum and the PWM controller allows the current to pass through C 2 only. When the frequency is high, the inductive reactance becomes a minimum, and the capacitor C 2 is inactive while C 1 is inserted. For the frequencies in between, both capacitors are switched. Before one switch turns on the other must first turn off allowing continuity of current flow. For that reason the resistor R SVC is integrated. The simulation used C 1 =1.689mF and C 2 =0.188mF. 6. Conventional rectifiers The simplest conventional rectifier is shown in Fig 5. It is commonly known as the uncontrolled diode bridge rectifier. 0 50 100 150 200 250 300 350 Electric frequency (Hz) Graph 1 Capacitive and Inductive reactance against electric frequency Figure 5: The uncontrolled diode bridge rectifier
This rectifier is able to convert a three-phase ac signal to dc, as it is made of three poles each one with two diodes. Two diodes from two poles are used to complete the current path in each cycle. The full wave diode bridge rectifier has very simple component arrangement but it is unable to control the PF. Full control of the diode bridge rectifier can be achieved by replacing the diodes with Thyristors shown in Fig. 6. We are able to control the output by selecting the proper fire angle and therefore higher power can be achieved. A full detailed description of the operation of both rectifier topologies is given in [9]. Figure 6: Fully controlled Thyristor bridge rectifier The same scheme of controlling the output as with the thyristor bridge rectifier but with a different technique can be achieved using a dc-dc boost converter. Fig. 7 shows the circuit of a diode bridge rectifier with boost converter which again can control the PF [5, 9] by controlling the on & off time of switch S. With this arrangement unity PF can be reached but the input current supply must be filtered using an LC filter. Figure 8: The PWM Voltage-Source Current-Controlled Rectifier This rectifier has bidirectional power flow and by applying very high switching frequencies we are able to minimise the low order harmonic contamination. The simulation results and an analysis of the performance characteristics of the seven rectifier circuits are presented in Section 4. 4. Simulation Results and Discussions The analysis considers the steady state speed at 3000 rpm for a 50kW SHEV application. Fig. 9 shows a comparison of the PF for all of the seven rectifier circuits for various load currents (load resistances). All CSCs achieve nearly unity PF, while the uncontrolled full wave diode bridge rectifier and the Thyristor rectifier demonstrate the lowest PFs. The PF of the diode rectifier with the boost converter drops significantly at relatively high load currents (small load resistances). SVC FCSC Figure 7: Diode rectifier with boost dc-dc converter The most common way to achieve unity PF at any desired output is by using a PWM rectifier. A PWM -source current-controlled rectifier is shown in Fig 8. The converter allows the control of the PF by controlling the phase currents [8]. Figure 9: Comparison of PF over load currents (load resistances) at an engine speed of 3.000rpm The study evaluated the impacts of the PF correction techniques in the output performance of the rectifier circuits and results are shown in Table 1 and Table 2. Table 1 illustrates the simulation results for the CSC circuits.
Table 1: Simulation results for an engine speed of 3.000rpm for the CSC FCSC SVC 4 89.87 4.355 0.999 493 3 117.5 3.343 0.999 487.5 2 170 2.331 0.999 476.8 1 308.9 1.306 0.999 448 4 92.53 4.075 0.999 497.9 3 122.2 3.059 0.999 493.9 2 180.1 2.039 0.999 485.9 1 343.2 1.011 0.999 463.5 4 88.44 5.43 0.998 497.6 3 123.1 3.49 0.997 493.4 2 182.3 2.19 0.997 485.3 1 342.5 1.09 0.996 462.6 Generally CSC circuits produce higher output s compared to conventional rectifiers but also a higher percentage of the THD of the source current. Fig. 10 shows a comparison of the output s of all seven rectifier circuits. Figure 10: Comparison of output over load resistances (load currents) at an engine speed of 3.000rpm The PWM rectifier can achieve high PF but its high switching frequency leads to high switching losses. CSCs can achieve almost unity PF without power degradation. The additional components, however, add switching losses and conduction losses. These losses will vary with the load current Table 2: Simulation results for an engine speed of 3.000rpm for the conventional rectifiers Diode Bridge Diode Bridge with Boost Thyristor PWM 4 58.8 3.97 0.82 316 3 65.8 3 0.83 266.2 2 73.0 2 0.84 179 1 78.7 1 0.856 106.4 4 92.99 0.48 0.99 380.8 3 81.7 0.479 0.975 290.9 2 73.25 0.46 0.85 196 1 69.59 0.36 0.5 98.3 4 59.34 3.92 0.823 312 3 66.16 2.97 0.834 263 2 73.08 2 0.845 195 1 67.7 21 0.48 100 Conclusion 4 71.3 1.6 0.9605 304.3 3 75.02 1.2 0.9687 238.3 2 78.17 0.8 0.9732 166.6 1 80.37 0.4 0.9739 85.5 The performance of rectifier circuits in generatorsets for SHEVs can be improved by adding CSC circuits. CSC will eliminate the effect of PF degradation caused by the inductance of the machine under all speed conditions. However, losses added by the additional components may offset the benefit of PF correction. Therefore, CSC converters applied to generator-sets will make only a contribution where the driving cycle is known e.g. in delivery vehicles. References [1] U. D. Choi, K. T. Kim, Y. N. Kim, S. H. Kwak, K. M. Kim, S. D. Lee, S. J. Jang, and K. Becksteard, "Development of the power generator for series hybrid electric vehicle," presented at 1st International Forum On Strategic Technology "e-vehicle Technology", IFOST 2006, Oct 18-20 2006, Piscataway, NJ 08855-1331, United States, 2006.
[2] S. G. Liddle, "Emissions from hybrid vehicles." Intersoc Energy Convers Eng Conf, 8th, Proc, Pap, Aug 13-17 1973, pp. 235-242, 1973. [3] A. Al-Busaidi, V. Pickert, Comparative study of rectifier circuits for series hybrid electric vehicles, presented at Hybrid & Eco-friendly Vehicle Conference 2008 (HEVC08), Warwick, UK, 2008. [4] D. Kalpaktsoglou and V. Pickert, "Controlled Series Capacitor Converters Applied to Wave Energy Conversion Buoys - A Simulation Study," presented at 4th IET Conference on Electronics, Machines and Drives. PEMD 2008, York, UK, 2008. [5] M. H. Rashid, Electronics: Circuits, Devices and Applications, 3rd ed. Upper Saddle River, N.J.: Prentice Hall, 2004. [6] N. G. Hingorani and L. Gyugyi, Understanding FACTS: concepts and technology of flexible AC transmission systems. New York: IEEE Press, 2000. [7] T. Miyasaka, K. Yamazaki, J. Tsuchiya, T. Shimizu, G. Kimura, and M. Shioya, "Improved operating characteristics of linear pulse motor using resonant current," presented at Proceedings of the 19th International Conference on Industrial Electronics, Control and Instrumentation, Nov 15-18 1993, Maui, Hawaii, USA, 1993. [8] J. R. Rodriguez, J. W. Dixon, J. R. Espinoza, J. Pontt, and P. Lezana, "PWM regenerative rectifiers: State of the art," IEEE Transactions on Industrial Electronics, vol. 52, pp. 5-22, 2005. [9] M. Kazerani, P. D. Ziogas, and G. Joos, "A novel active current waveshaping technique for solid-state input power conditioners," IEEE Transactions on Industrial Electronics, vol. 38, pp. 72-78, 1991.